This invention relates to a molten core retention system for a liquid-metal-cooled fast breeder reactor. In more detail, the invention relates to an in-vessel safeguard baffle assembly designed to prevent molten fuel from melting through the containment vessel for the reactor. Such a retention system is frequently called a "core catcher" in the literature and for brevity this name will be employed hereinafter.
It is postulated that under certain conditions an accident could occur to a fast breeder reactor which could cause all or part of the fuel to melt. One example of a sequence of events that could lead to a release of radioactive material can be hypothesized as follows:
A. Objects used during plant construction are forgotten and left in the piping system. PA1 B. Sodium flow during plant operation causes the objects to move downstream to the core inlet region. PA1 C. One section of the core is starved of coolant flow because of the blockage by foreign objects. PA1 D. Fuel pins melt in the region of starved flow. PA1 E. Molten material flows downward by gravity from the core region and collects on the reactor vessel bottom. PA1 F. The reactor vessel bottom melts as a result of the heat generated by the debris collected thereon. PA1 G. Sodium and radioactive debris flow from the reactor vessel into the guard vessel by gravity. PA1 H. The guard vessel bottom melts as a result of the heat generated by the radioactive debris collected thereon. PA1 I. Sodium and radioactive debris flow through the melted hole in the guard vessel to contaminate the biosphere. PA1 1. In-reactor vessel core catchers. PA1 2. In-reactor tank core catchers. PA1 3. Sodium-cooled trays in the reactor cavity. PA1 4. A cooled reactor cavity liner. PA1 5. A sacrificial bed within or outside of the reactor vessel. PA1 a. Space for containment of substantial quantities of radioactive debris material (possibly greater than 50% of the core) must be provided. PA1 b. Criticality of the contained debris must be prevented. PA1 c. The heat generated by the debris must be dissipated without overheating the containment structure. PA1 d. The debris material must be adequately cooled by natural means -- such as natural circulation of sodium. PA1 e. The containment structure must not introduce unacceptable resistance to sodium flow. PA1 f. The containment structure must withstand normal operating loads, anticipated transients and unlikely plant failure events without damage, for extended periods of operation. PA1 g. Manufacturing must be performed using conventional techniques.
Clearly such a sequence of events -- or any other sequence of events leading to contamination of the biosphere with radioactivity, even though the probability of occurrence is very low -- cannot be tolerated. Various design approaches have been proposed to solve this potential problem.
These include:
A number of different attempts have been made to solve the problem with an in-vessel core catcher but these attempts were deterred from finding a practical solution by the inability to satisfy one or more of the following requirements: